This invention relates to alphanumeric displays and more particularly to a multicolor alphanumeric display. Still more particularly the invention is related to a multicolor monolithic light emitting diode display which is scannably addressable to produce characters in alternative or simultaneous colors.
As pointed out in the co-pending application of Michael G. Coleman and John A. Keil, Ser. No. 198,221 filed Nov. 12, 1971, and assigned to the present assignee, visual readout devices such as alphanumeric displays, are utilized for many purposes such as computer readouts, process control instrumentation, aircraft and automotive instrument panels and various other indicators such as clocks and gauges. In many of these uses it would be desirable if the display could operate in a differential color mode so as to provide an additional indicator of the meaning of the information. For example, a direct reading altimeter for an aircraft might be programmed so that altitudes below a certain height might appear in a distinctive color to further warn the pilot of the position of the aircraft. Also, since the multicolor basically can occupy the same panel space as a single color display two distinct types of information may be displayed in the same panel space by the use of one color to indicate one type of information and another color to indicate still another type of information. Thus, a multicolor display can occupy less space and provide additional functions not presently available from standard display devices.
The practical realization of the matrix display of this type requires precision photolithography as developed for semiconductor integrated circuits and silicon material by planar processing. While the aforementioned patent application results in a useful multicolor display, it does require the use of liquid epitaxy process to grow efficient red light emitting gallium phosphide (Zn-O) diodes. Liquid epitaxial processes do not readily produce a precision, flat epitaxial growth. Thus, it is preferred to utilize with the accordance of the invention an alternative material, gallium arsenide phosphide, for the red emitting diodes which material can be grown by a vapor epitaxial process. Further, in any matrix display which is directly addressable from logic and which has no intrinsic memory in the display itself the total display must be scanned faster than the eye responds.
Obviously if there are several thousand elements in the display then the time in which current flows through a given segment is short to obtain reasonable brightness and extremely high current level must be passed through the segment during this short address time. If this type of time sharing is to be successful in large displays it is essential that the light output be proportional to the current so that the pulse brightness during the short address time is usefully high. Red light emitting gallium phosphide (Zn-O) suffers from the defect that it saturates at modest current level. Thus, it is desirable again to use the gallium arsenide phosphide as the red emitting material because this material has a characteristic wherein the brightness is proportional to current up to very high current levels.